When a person's heart does not function normally due to, for example, a genetic or acquired condition, various treatments may be prescribed to correct or compensate for the condition. For example, pharmaceutical therapy may be prescribed for a patient or a pacemaker may be implanted in the patient to improve the function of the patient's heart.
In patients with severe heart problems cardiac resynchronization therapy (“CRT”) may be prescribed. Briefly, CRT involves attempting to resynchronize the actions of the chambers of the heart. Thus, CRT may be prescribed for patients with significant atrioventricular mechanical dys-synchrony (“DYS”), interventricular mechanical DYS, or intraventricular mechanical DYS. As an example, a bundle branch block may disrupt the normal synchronized depolarization of the left and right ventricles. CRT may attempt to address this problem by, for example, pacing the left and right ventricles at substantially the same time.
One central issue in CRT is identification of patients most likely to respond to the therapy. Conventionally, a wide QRS complex has been correlated with mechanical ventricular DYS. Accordingly, a significant percentage of the controlled studies on CRT have been conducted on patients suffering from congestive heart failure that have a wide QRS complex.
While the above correlation may be correct in some instances, it is not true in all cases. For example, some patients with a wide QRS complex do not have marked mechanical ventricular DYS. Conversely, some patients with a normal or narrow QRS complex may still suffer from significant mechanical DYS and, hence, are candidates for CRT. Moreover, a surface ECG measured for CRT assessment may not be accurate since the ECG is affected by the insulted location of the heart. Also, although some short-term experimental studies have shown that patients with wider QRS complexes have a greater immediate mechanical response to CRT, a significant percentage of long-term studies have shown that QRS complex duration does not predict a response to CRT. In addition, narrowing of the QRS complex may not predict a functional improvement following CRT. Furthermore, some CRT recipients may experience a worsening of symptoms and mechanical DYS. In summary, a significant percentage of patients selected for CRT based on QRS complex duration as a surrogate for mechanical ventricular DYS may not respond to the therapy.
Moreover, some methods for determining whether a patient is responding to CRT are relatively expensive. For example, a series of relatively expensive echocardiogram procedures may be used to measure any changes in the patient's cardiac output over time. Such a procedure is typically performed by a doctor in the doctor's office or a clinical setting, further adding to the overall cost. Furthermore, such a procedure may not reflect real-time conditions such as when the patient is exercising or walking up a flight of stairs.